1. Field of the Invention
The present invention relates to a high electron mobility transistor (HEMT) for use in a high-frequency amplifier or the like.
2. Description of the Prior Art
A field-effect transistor (FET) having a superlattice structure which was first disclosed by Mr. Mimura of Fujitsu Limited. is known as HEMT (see U.S. Pat. No. Re. 33,584), and is widely used as an essential circuit component in portable telephone sets.
Although the conventional HEMT disclosed by Mr. Mimura has excellent high-frequency characteristics, it suffers a limitation as to frequencies which it can handle, and cannot be used in a millimeter-wave band of frequencies ranging from 60 GHz to 94 GHz.
In view of a report issued by the FCC (Federal Communications Commission) in the U.S.A. on the allocation of automotive millimeter-wave radar frequencies, the assignee of the present application has launched a project for the development of a semiconductor that can be used in a 77 GHz band of frequencies for use by automotive millimeter-wave radars in the U.S.A. The HEMT structure dislosed by Mr. Mimura is unable to handle such a 77 GHz frequency band. As a result, there has been proposed a pseudomorphic-HEMT which employs InGaAs for an FET channel for improved frequency characteristics as disclosed in U.S. patent application Ser. No. 08-565295 filed on Nov. 30, 1995.
The above U.S. patent application reveals a high-performance high electron mobility transistor which has a channel layer whose thickness is limited to a value small enough to substantially uniformize the density of an electron gas in the transverse direction of the channel layer, and which also has upper and lower wide-band-gap layers comprising AlGaAs layers of high resistance. With the disclosed structure, the HEMT has its mutual conductance variable to a small degree depending on the gate voltage.
Specifically, if the channel layer is thick, then a two-dimensional electron gas layer developed in the channel layer is localized in the vicinity of the heterojunction, and hence divided into two layers positioned at different depths from the surface, i.e., spaced from the gate electrode by different distances. These two electron gas layers are affected differently by the gate voltage. As a result, it has been considered that the mutual conductance depends largely on the gate voltage.
According to the invention disclosed in the above U.S. patent application, the thickness of the channel layer is limited to such a value, specifically in the range of from 50 .ANG. to 150 .ANG., as to regard the density of the electron gas in the channel layer as being substantially uniform, and the resistance of the upper and lower AlGaAs layers which are positioned adjacent to the thin channel layer is of a high value. The resistance of the upper and lower AlGaAs layers is high because as it increases, the effect of the gate voltage applies to a wider range including the channel layer, resulting in the same advantage as would be achieved if the channel layer thickness were reduced.
FIG. 1 of the accompanying drawings shows experimental data on electric characteristics of the high electron mobility transistor disclosed in the above U.S. patent application. The experimental data show the relationship between the drain voltage and the drain current when the gate voltage was varied stepwise. It can be seen from FIG. 1 that the drain current increases in a substantially uniform pattern as the gate voltage increases, and hence the mutual conductance does not vary largely depending on the gate voltage.
FIG. 2 of the accompanying drawings is a graph showing experimental data on the relationship between the mutual conductance and the gate voltage of the high electron mobility transistor disclosed in the above U.S. patent application and other conventional high electron mobility transistors. The vertical axis of the graph represents the mutual conductance (gm) per unit gate width and the horizontal axis the gate voltage. A solid-line curve A indicates the data of the high electron mobility transistor disclosed in the above U.S. patent application. A dotted-line curve B indicates the data of a conventional high electron mobility transistor. A dot-and-dash-line curve C indicates the data of an improved conventional high electron mobility transistor. The experimental data shown in FIG. 2 support the effectiveness of the high electron mobility transistor disclosed in the above U.S. patent application.
In the high electron mobility transistor disclosed in the above U.S. patent application, the electron gas produced in the channel layer is both physically integrated by the thickness of the channel layer and functionally integrated by the high resistance of the AlGaAs layers adjacent to the channel layer, providing the good mutual conductance characteristics shown in FIGS. 1 and 2. However, since the resistance of the upper AlGaAs layer over the channel layer is increased for improving the mutual conductance characteristics, the gate electrode imposes an excessive effect on the channel layer, tending to cause the threshold voltage to be of too a small negative value.